This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. This protocol will explore the effects of hyperoxia and increased sympathetic nerve activation of coronary blood flow in healthy volunteers. 15 Subjects male or female between ages 21-45y. Free of acute or chronic disease and off all medications. The study will involve three visits, one to determine baseline response of coronary blood flow to hyperoxia (after breathing 100% O2 x 10 minutes) followed by the effects of increasing lower body negative pressure (which increases sympathetic nerve activation). The following two visits will recreate these conditions, but antagonize the physiologic response through antagonists. The effects of increased SNA will be antagonized by IV administration of phentolamine, an alpha 1 adrenergic receptor antagonist, in the second visit and the effects of hyperoxia will be challenged with IV Vitamin C in the third visit. Coronary blood flow will be determined by echocardiography Doppler flow. Blood will be drawn to measure a prostaglandin at baseline and after intervention. Heart rate and blood pressure will be monitored continuously.